Differential Kinetic Analysis of Mixtures Using a Continuous Flow Method

SIR: The practical range for the study of reaction velocities has been extended to time values as small as 0.001 second by the application of continuous flow methods. The principles of the flow methods were introduced by Hartridge and Roughton in 1923 ( 7 ) and form the basis for the methods available today. A detailed discussion of these methods has been given recently ( I O ) . Basically, the methods consist of allowing t a o solu- tions, which are to react, to flow to- gether in a mixing chamber and then through a reaction tube, where the progress of the reaction is observed. With constant velocity, j , maintained through the tube, the reaction is always in the same state when the solution reaches a given point. The time, t , for this state of reaction is then cal- culated from the volume, V , of the tube up to this point, using

t = v/j (1)

The following discussion shows how these continuous flow principles for fast reactions can be applied to the dif- ferential kinetic method of analysis for mixture.

The procedure is illustrated by examples of reactions which liberate iodine. A known concentration of thiosulfate, less than that required to consume all the iodine produced, is included in the reaction mixture along with starch indicator. The point a t which all the thiosulfate is consumed and free iodine is first present appears as a color boundary in the observation tube, The iodine-thiosulfate reaction is extremely rapid and does not enter into the final calculation. By successively changing the thiosulfate concentration, the time of reaction for different frac-

tions of the initial sample concentration can be measured and the normal re- action rate plots can be made. For a mixture of two components reacting with iodide, the procedure i:, run using as many different concentrations of thiosulfate as there are points needed to define the final linear portion of the rate plot. Since the iodide ion is kept constant by the reaction of thiosulfate with the iodine produced, the reaction is treated as a pseudo first-order process.

EXPERIMENTAL

Apparatus. The apparatus, illus- trated in Figure 1, is t h a t of Haggett, Jones, and Oldham (6) with minor al- terations. I t consists of two reactant solution reservoirs, 100-ml. burets, con- nected to a mixing chamber which is a three-way, T-bore stopcock (1-mm. bore). The flow tube (2-mm. bore) is about 1 meter long and is attached to a meter stick. The volume of the flow tube is calibrated for different points along its length with mercury. A plot is prepared of milliters f ig . centimeters and a smooth curve is drawn.

To measure the flow rate through the tube, both burets are filled with water and, with stopcocks A , B , and C open and D closed, vacuum is applied by means of a water aspirator. Stopcock D is opened and a stop watch started a t the instant of opening. rifter about 90 ml. of water have drained from each buret, stopcock D is closed and the time is noted. The l e ~ e l of each buret is read, and the total flow rate, milli- liter per second, is calculated. If the levels in the two burets differ, stop- cocks .I and B are adjusted to remove the difference.

Procedure. To one buret is added a solution containing about 0.035 equivalent per liter of sample, 200 ml. of starch solution per liter, and a known amount of sodium thiosulfate.

The amount of thiosulfate used is changed from run to run. The quan- t i ty of thiosulfate added is in the range necessary to bring the color boundary along the measurable por- tion of the reaction tube but , in any case, is less than tha t \\hich would consume all the iodine liberated.

To the other buret is added a solution about 0.15,V in potassium iodide and about 0.5.V in hydrochloric acid. With stopcocks A , B. and C open and D closed, vacuum is applied to the system. Stopcock D is then opened and the dis- tance between the mixing chamber and the color boundary in the reaction tube is measured.

The total concentration of the sample is determined iodometrically.

Calculations. The volume, V, is read from the calibration curve, using the distance measured to the color boundary in centimeters. The time, t , of the partial reaction is calculated, letting j equal the flow rate

t = v/j The reaction is treated as pseudo

first order, and the graphical method of Siggia, Hanna, and Serencha (la) is applied. Log (a - 2) is p l o t t a against t , where a is the original con- centration and z is the concentration reacted in time, t , as measured by the thiosulfate initially present. If two components are present, each reacting a t a different rate, two slopes are &- tained. When the second slope is ex- trapolated to zero time, a t the point of intersection y, z = al, the concentration of the faster reacting component and

(2) log (a - Ul) = y

This equation can be solved for al.

M e t e r stick

-2.8 1 I I I I I 0 0.02 0.04 0.06 0.08 0.10

to v a c u u m

Figure 1 . Apparatus

2022 ANALYTICAL CHEMISTRY

Seconds

Figure 2. Reaction rate curve for mixture of potassium dichromate and potassium bromate

RESULTS A N D DISCUSSION

The data in Table I show the results obtained, and the rate plot obtained for the mixture of potas#jium bromate and

sium dichromate is shown in Figure ince chlorate does not react with

iodide, it does not, interfere in the differential analysis of the iodate and bromate contents of ternary mixtures of these compounds. Determining the total halogen content of such mixtures and obtaining the chlorate by difference

tem of analysis for all three components.

The purpose of this communication is to demonstrate the applicability of flow methods to the analysis of mixtures. I t should be pointed out that measure- ment techniques other than those used here most probably ‘can be applied to differential kinetic a.nalysis using flow systems. Blaedel and Olson ( 2 ) re- cently described an amperometric meas- surement technique in a flowing system which they applied to the analysis of glucose based on an enzymatic oxidation reaction. These workers measured the amount of reaction occurring between two tubular platinum electrodes in the flowing stream and, by comparison with the amount of reaction in solutions of known concentration, determined t’he concentration of glucose in unknown samples. Other techniques which have been applied to measure reaction velocities are conductance and electro-

A Potassium

iodate Potassium

iodate Potassium

periodate Potassium

dichromate

Table I. Analytical Results for Mixtures

Mixture ~- R

Potassium bromate

Potassium bromate

Pot assiuni bromate

Potassium bromate

5 Faster Reacting Component, A

C Found Present 24 3 24 8

56 2 Potassium 13 8 14 2

56 8, 57 3

chlorate 9 5 9 6

2 9 . 2 30.6

motive force measurements (11) , temperature rise in exothermic reactions ( I , 8, 9 ) , polarographic measurements (3 , 4), and si)ectrophotometri(, measure- ments (5, I S ) . Most of these techniques are 1-ery rapid since the measurements can be made a t many points along the observation tube and therefore require only one run. They do require specialized instrumental arrangements.

LITERATURE CITED

(1) Baternan, J. B., Roughton, F. J. IT,, (2 ) Blaedel, W. J., Olson, C., ANAL.

Biochem. J . 29, 2622, 2630 (1955).

CHEM. 36, 343 (1964).

H.. Acta Chem. Scand. 1. 685 (19471. (3) Bonnichsen, R., Chance, B., Theorell,

( 4

Chern. 199, 383 (1952). (5

(6) Haggett, 11. L., Jones, P., Oldharn, K. B., J . Chern. Educ. 40, 367 (1963).

) Chance, B., Bwchem: J . ‘46, ’387 (1950). ) Chance, B., Seilands, J. B., J . B id .

( 7 ) Hartridge, H., Roughton, F. J. W.,

( 8 ) La Mer, V. K., Read, C. L., J . A V I . Proc. Roy. Soc. A104, 376 (1923).

Chem. Soc. 52, 3098 (1930). (9) Pinsent, B. R. R., Pearson, L.,

Roughton, F. J. W., Truns. Faraday soc. 52, 1512, 1594 (1956).

(10) Roughton, F. J. W., Chance, B., “Techniques of Organic Chewistry,” Vol. VIII, 2nd ed., Chap. XIV, S. L. Fries, E. S. Lewis, A. \Veissberger, eds.. Interscience. Sew York. 1963.

(11) Saal, R. ?;. J.)’ Rec. ‘I’rav. C‘hzm. 47, 7 3 . 264 (1928). \ - -,

(12) ’Siggia, S., Hanna, J. G., Serencha,

(13) Theorell, H., Chance, B., Acta ?;. AI., h . 4 L . CHEM. 35, 575 (1963).

Chem. Scand. 5, 112i (1951). J. GORDON H a m a

SIDNEY SIGGIA Olin Research Center Olin Mathieson Chemical Corp. 275 Winchester Ave. Sew Haven 4, Conn.

RECEIVED for review April 13, 1964. Accepted June 24, 1964.

Elec troc hemica I Studies Using Con d uc ti ng Glass Indicator Electrodes

SIR: The n-type, broad band (6) semiconductor property of tin oxide has been utilized for seveiral years to make semiconduct,ing glass surfaces. These tin oxide-coated glasses have been used for a potentiometric electrode by Cooper (3) and for p1ai;ing and deplating of ailver on glass by Xtn te l l and Zaromb (?). 1Te wish to report the use of such a conducting glass electrode (CGE) as a working indicator electrode for the oxidation and reduction of inorganic ions as lvell as organic molecules in both acidic and basic solutions. The ad- vantage of CGE is i,ts optical t,rans- parency in the 3050- to 7000-A. r of the spectrum which makes po the spectral monitoring of certain ab- sorbing electroactivr species, inter- mediates, or products concurrently during an electrolysis. The application of CGE to the spectral monitoring of an electrode process is herein demonstrated.

EXPERIMENTAL

Apparatus. Tin oxide-coated glass surfaces were obtained commercially from Libbey Owens Ford Glass Co. Pret,reatment prior to use consisted of washing the surface with reagent grade benzene to remove adherent organic adhesive materials followed by light rubbing of the surface with wetted, 1000-mesh silicon carbide powder, and t,lien thoroughly washing with distilled water. S o special pre- cautions or pretreatments were neces- sary hetween runs either in acidic or alkaline medium. When not in use, t.he electrodes were stored dr

For chrono1)otentiometry ammetry studies. cells were constructed with the conducting mrface in a narrow ring configuration to maximize the ratio of surface area to surface resistance. For spectral studies during electrolysis, cells were made from cither one or two flat Illates of conducting glass. Further details of cell construction and con-

figuration will be discusse in a future paper.

The circuits for the construction of triangular wave generator and potentio- stat were identical to those described by .Ilden, Chambers, and .‘idams ( 2 ) . The chronopotentiometric circuit and equipment were conventional in all respects. An Electro-Instruments Model l O O T 1 3 X-I.’ recorder and Leeds 8: Sorthrup Model H strip chart re- corder were wed. XI1 potentials are reported with respect to a saturated calomel reference electrode (S.C.E.)

Reagents. Xnthraquinone was purified according to published pro- cedure ( 5 ) . Eastman Iiodak TThite Label Grade o-tolidine was used without further purification. Standard ferrous ammonium wlfate (99.85Oj,) obtained from Thorn Smith Co., Royal Oak, l I ich. , was used for the source of ferrous ion. All other chemicals were of reagent, grade. Distilled water was redistilled from alkaline permanganate solution.

VOL. 36, NO. 10, SEPTEMBER 1964 2023